It has become demanded to enhance reliability by further improving various performances, such as heat resistance, mechanical properties, chemical property, and electrical property, in the electronic or electrical equipment (hereinafter, which may be simply referred to as electrical equipment) developed in recent years, as compared to the conventional electrical equipment. Under the situations, excellent abrasion resistance, thermal aging resistance property, and solvent resistance have become required for insulated wires, such as enameled wires, that are used as magnet wires for electrical equipment for aerospace use, electrical equipment for aircrafts, electrical equipment for nuclear power, electrical equipment for energy, and electrical equipment for automobiles.
Further, advance of electrical equipment represented by motors or transformers has been progressed resulting in size reduction and improved performance. Thus, it becomes usual in many cases that insulated wires are used in such a way that the insulated wires are worked by winding (also referred to as coil working) to winding wires and they are pushed into a quite small space to pack. Specifically, it is no exaggeration to say that the performance of a rotating electric machine, such as a motor, is determined by how many wound wires produced by coil working the insulated wires into a coil shape can be held in a stator slot. As a result, improvement of the ratio of the sectional area of conductors to the sectional area of the stator slot (space factor) has been required to be particularly highly increased.
It has been lately attempted to use a rectangular wire in which the conductor has a shape similar to a quadrilateral (square or rectangle), as a means for increasing the space factor.
However, while use of a rectangular wire exhibits a dramatic effect in increasing the space factor, a corner portion of cross-section rectangular is extremely brittle to bending working, such as coil working. Due to this, there is a problem that a coated film cracks when the rectangular wire is worked by applying strong pressure. In particular, it is known that the crack of the coated film tends to occur more easily as the curvature radius of the corner portion is smaller.
As the insulated wire using the rectangular wire, for example, Patent Literature 1 proposes an insulated wire having a high partial discharge inception voltage and an improved insulating property of the corner portion thereof, by bearing a predetermined relationship to each of thicknesses and dielectric constants with respect to the corner portion and the side portion of the insulating film. However, in order to increase the partial discharge inception voltage, thickening of the insulating film is needed, which results in lowering of the space factor.
Further, Patent Literature 2 discloses insulated wires having a suppressed gap between the insulated wires and an improved space factor, by inflecting a cross sectional shape of the insulating film thereof.